The present invention relates generally to a method for reaming a hole in a metal substrate as part of a finishing operation. The present invention further relates to an improved reamer for carrying out this method.
In a wide variety of drilling, repair or reconditioning processes for metal substrates, such as shafts, flanges, compressor disks and other turbine engine components, frames, casings and engine mounts, it is necessary to accurately or precisely cut or finish the hole that is formed to a predetermined diameter. For example, in many cases after a drilling operation involving a metal substrate, reaming of the hole is carried out to impart a finer finish thereto. See Tonshoff et al, xe2x80x9cTool and Process Design for High-Efficient Machining,xe2x80x9d Soc. Mfg Eng., (1999), MR99-255-9. A conventional reamer used in such reaming operations generally has a cylindrical body, a shank at one end of the body, typically four, six or eight helical or spiral grooves or flutes formed in the outer peripheral surface of the body and a chamfered leading end opposite the shank end where the flutes terminate. Each flute also has a chamfer face proximate its chamfered leading end. See U.S. Pat. No. 5,921,728 (Kammeraad et al), issued Jul. 13, 1999. See also U.S. Pat. No. 4,231,693 (Kammeraad), issued Nov. 4, 1980.
As the reamer rotates during a typical reaming operation, the leading cutting edge (sometimes referred to as a xe2x80x9ccutting toothxe2x80x9d) of each chamfer face axially cuts or scrapes the surface of the hole of the metal substrate. The metal cuttings or scrapings that are generated from the surface of the hole during this reaming operation are commonly referred to as xe2x80x9cchips.xe2x80x9d These metal chips can be formed in various sizes, shapes and configurations and are deflected away from the cutting zone of the reamer by the chamfer face. The angle that the chamfer face makes, relative to plane at the chamfered end perpendicular to the rotational axis of the reamer, is typically about 45xc2x0 for a standard reamer.
During the reaming operation, the reamer is typically advanced into the hole at a particular penetration feed or advance rate. This advance rate is typically measured in terms of the longitudinal distance (in mils or thousandths of inch or mm) the reamer advances or penetrates into the hole, per rotation or revolution of the reamer. Because reamers can have different numbers of flutes, and thus different numbers of cutting edges or teeth, a more uniform value for this advance or feed penetration rate is based on the distance the reamer advances or penetrates into the hole per cutting edge or tooth. This is usually determined by taking the advance or feed penetration rate per revolution and dividing it by the number of cutting edges or teeth on the reamer.
For standard reaming operations, the advance or feed penetration rate of the reamer (for example, a reamer having eight flutes, and therefore eight cutting edges or teeth) is typically about 8 mils (0.20 mm) per revolution or about 1 mil (0.025 mm) per cutting edge or tooth. At this standard advance or penetration rate, relatively long and stringy metal chips are typically formed. These long and stringy metal chips are not easily deflected away from the cutting zone, but instead tend to collect around the reamer and form a xe2x80x9csteel wool-likexe2x80x9d mesh or mass of material that surrounds the outer periphery of the reamer. This generation of the xe2x80x9csteel wool-likexe2x80x9d mesh or mass of material becomes even more acute as the length of the hole to be reamed becomes longer, especially relative the outer cutting diameter of the reamer.
The collection of this xe2x80x9csteel wool-likexe2x80x9d mesh or mass of material around the outer periphery of the reamer can create a number of problems. Frequently, fluid coolants need to flow or be fed to the cutting zone of the reamer to lower its temperature as heat is generated during the reaming operation. Indeed, it can be extremely important to have this coolant flow unimpeded to the cutting zone to avoid excessive wear or damage to the reamer. Unfortunately, as this xe2x80x9csteel wool-likexe2x80x9d mesh or mass of metal chips forms and surrounds the outer periphery of the reamer during a standard reaming operation, the flow of coolant to the cutting zone of the reamer can be greatly impeded. This can lead to undesired wear or damage to the reamer, as well as making accurate and precise finishing of the hole difficult to achieve.
Accordingly, it would be desirable, therefore, to provide a method for reaming a hole that does not generate this xe2x80x9csteel wool-likexe2x80x9d mesh or mass of metal chips that can surround the outer periphery of the reamer and thus impede the flow of coolant to the cutting zone, especially when the length of the hole is relatively long relative to the outer cutting diameter of reamer. It would further be desirable to provide an improved reamer that minimizes the undesired generation of this xe2x80x9csteel wool-likexe2x80x9d mesh or mass of metal chips when carrying out this reaming operation.
The present invention relates to a method for reaming a hole in a metal substrate with a reamer that includes a body having a rotational axis, at least three flutes formed in the body, each flute terminating at a chamfered leading end and having a chamfer face proximate the chamfered end, the chamfer face having a leading cutting edge. This method comprises longitudinally advancing the chamfered end of the reamer into the hole at the rate of at least about 5 mils (0.13 mm) per cutting edge as the hole is reamed during rotation of the reamer. For holes having a length (L) that is at least about 3 times the cutting diameter (D) of the reamer, a preferred subsequent step is to reduce the rate of advance of the chamfered end of the reamer into the hole to about 1 mil (0.025 mm) or less per cutting edge for at least about 1 revolution or rotation of the reamer.
The present invention further relates to an improved reamer for carrying out this reaming method. This improved reamer comprises:
(a) a body having a rotational axis;
(b) at least three flutes formed in the body;
(c) each flute terminating at a chamfered leading end and having a chamfer face proximate the chamfered end;
(d) the chamfer face having a leading cutting edge and a chamfer angle of from about 10 to about 300;
(e) each flute having a negative radial rake angle.
The reaming method, as well as the improved reamer, of the present invention, provides a number of significant advantages and benefits over standard reaming methods and reamers. By increasing the rate of advance or feed penetration of the reamer during the reaming operation, the tendency during reaming to create long and stringy metal chips that result in a xe2x80x9csteel wool-likexe2x80x9d mesh or mass of material that can surround the outer periphery of the reamer is either minimized or avoided. As a result, coolant flow to the cutting zone is impeded less or not at all, thus avoiding excessive wear and damage to the reamer. By avoiding excessive wear and damage to the reamer, more holes can be finished in less time by minimizing the number of times the reamer needs to be changed. The preferred subsequent step of momentarily reducing the rate of advance of the chamfered end for holes having higher L/D ratios relative to the outer cutting diameter of the reamer increases the likelihood that the metal chips will break off before they become excessively long, again avoiding the formation of the xe2x80x9csteel wool-likexe2x80x9d mesh or mass of material around the outer periphery of the reamer. The improved reamer where each flute has a chamfer face with a decreased or xe2x80x9cshallowerxe2x80x9d chamfer angle provides an improved pathway for easier and quicker removal of chips from the cutting zone of the reamer, especially when used in reaming operations involving long or deep holes. An optional but preferred aspect of this improved reamer is to provide a rounded or xe2x80x9chonedxe2x80x9d cutting edge for the chamfer face. Rounding or xe2x80x9choningxe2x80x9d of the cutting edge improves its strength to resist chipping, as well as polishing the grind line to improve the finish or edge of the improved reamer.